Alopecurus myosuroides (black-grass) is the
major herbicide-resistant weed problem and, by 2013, occurred on virtually all
of the estimated 20,000 farms in 35 counties where herbicides are applied
regularly for its control. Resistance to mesosulfuron + iodosulfuron, first
used in the UK in autumn 2003, has now been detected on >700 farms in 27
counties in England. Resistance is conferred by both ALS target site (Pro-197
& Trp-574 mutations) and non-target site mechanisms. Resistant Lolium multiflorum (Italian rye-grass) occurs
on >475 farms in 33 counties and resistant Avena
spp. (wild-oats) on >250 farms in 28 counties of England. The first
cases of ALS target site resistance (Pro-197) in UK populations of L. multiflorum were detected in 2012.
ALS-resistant Stellaria media (common
chickweed) was found on >50 farms in 13 counties in England, Scotland and
Northern Ireland and ALS-resistant Papaver
rhoeas (common poppy) on >40 farms in nine counties of England. ALS-resistant Tripleurospermum
inodorum (scentless mayweed) was found on five farms in three counties
(Yorkshire, Norfolk and Angus). These included the first recorded case in
Scotland where the ALS mutation responsible (Pro-197-Gln) was determined,
making this the first UK population of Tripleurospermum
inodorum to have ALS target site resistance confirmed.